The present invention relates to an image forming apparatus, such as a laser printer, a digital copy machine, etc., for example, and it also relates to an optical beam scanning device, being applicable into such the image forming apparatus.
For dealing with speed-up or high-speed of the optical beam scanning devices, the sound of wind noises generated by a polygon mirror come to be large if increasing the rotation speed of a polygon motor, and an amount of heat generation therein also increases. For reducing such the drawbacks, it is effective to provide on the polygon mirror a cover for sound insulation or cooling thereof. Further, the cover also can achieve a function of protecting the reflection surfaces of the polygon mirror from adhesion of dusts thereupon. When providing the cover, it is necessary to provide a window (i.e., a cover glass) for allowing a light beam to enter and emit therethrough. Although treating an anti-reflection coating upon the cover glass, it is impossible to reduce the reflection thereupon down to zero (0), completely; therefore, still the reflection light is generated thereon.
This reflection light, impinging or striking upon a flange portion or an edge portion of a scanning optical system element (i.e., a lens and a mirror, etc.), reaches an image surface as a fixed stray light, thereby bringing about a possibility of exerting a bad influence upon a copying image and/or a printing image formed thereon.
For solving such the drawback due to the reflection light, there is already described a method in Japanese Patent Laid-Open No. 9-274134 (1997), for example. In the method described therein, a cover glass disposed surrounding the polygon mirror is so inclined that the reflection light can escape into the sub-scanning direction, for the purpose of removing such the stray light therefrom. Thus, since a scanning line is curved into the sub-scanning direction on the surface to be scanned with that inclination, a correction flat plate is also provided, for correcting or compensating the curve of the scanning line, which is generated on the cover glass.
However, with this method, the cover glass is inclined, resulting in deteriorating or degrading the optical characteristics, and further there is added a correction flat plate for correcting or compensating that. With this flat plate, though it is possible to make the correction or compensation, it brings about a drawback of increasing the number of parts thereof, and also there may be a possibility that the optical characteristics thereof are lowered due to an increase of optical surfaces therein, even thought it is the flat plate.